A Russian ceasefire monitoring center in Syria says more than 2,000 militants have gathered in a northern neighborhood of the embattled city of Aleppo.

In a statement, the center said Saturday that "militants from different terrorist organizations and groups of the so-called 'moderate opposition' are concentrated in the region of Sheikh Maqsood.”

Quoting civilians in Aleppo, the center said armed groups, partly made up of Turkish soldiers, had appeared north of the city and begun an offensive against positions held by Syrian troops southwest of Aleppo.

An official with the monitoring base said more than 40 people were killed and about a hundred injured in a shelling by the al-Qaeda-affiliated al-Nusra Front terrorist group on Saturday.

"There was continuous bombardment by rocket launchers, canon artillery, mortars and anti-aircraft installations. The terrorists shelled several areas of the city inhabited not only by government forces and Kurdish troops but also civilians," Russia’s TASS news agency quoted the official as saying.

Syrian security forces and residents gather near a building that was hit by militant shelling as they search for survivors in the Hamdaniyeh neighborhood, Aleppo, June 4, 2016. (AFP)

A nation-wide cessation of hostilities, brokered by Moscow and Washington, was introduced in February in a bid to facilitate dialogue between Syria’s rival parties. ISIS and al-Nusra Front terrorist groups were excluded from the truce.

However, renewed violence in some parts of Syria, particularly around Aleppo, has left the ceasefire in tatters in recent weeks and torpedoed peace talks on the conflict.

Earlier this week, Russian Foreign Minister Sergei Lavrov said in televised remarks that Washington has asked Moscow not to target the Nusra Front.

He called on the so-called moderate opposition groups, supported by the US, to leave the areas held by al-Nusra extremists.

"They (the US) are telling us not to hit it (Nusra), because there is 'normal' opposition next to it," Lavrov said.

On September 30, 2015, Russia launched its air campaign against ISIS and other terrorist groups upon a request by the Damascus government. Later in mid-March, the bulk of Russian military forces were withdrawn from Syria.